Proof, Persuasion, and the Galileo Affair
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Plenary Presenters Truth in Science: Proof, Persuasion, and the Galileo Affair Truth in Science: Proof, Persuasion, and the Galileo Affair Owen Gingerich In 1616 in a letter destined for Galileo, Cardinal Roberto Bellarmine (the leading Catholic theologian of his day) expressed his doubts about finding evidence for a moving earth. Would the annual stellar parallax or the Foucault pendulum have convinced him? The historical setting explored in this essay suggests that the cardinal would not have been swayed by these modern “proofs” of the heliocentric cosmology, even though they are convincing to us today because in the meantime, we have the advantage of a Newtonian framework. What passes today for truth in science is a comprehensive system of coherencies supported more by persuasion than “proofs.” What kind of n April 12, 1615, Cardinal Roberto in the absence of an apodictic proof when OBellarmine, the leading Catholic he added: evidence theologian, wrote an often-quoted If there were a true demonstration, convinced letter to Paolo Antonio Foscarini, a Carmelite then it would be necessary to be very monk from Naples who had published careful in explaining Scriptures that Galileo and a tract defending the Copernican system. seemed contrary, but I do not think Bellarmine’s letter, which was obviously Kepler that the there is any such demonstration, since intended as much for Galileo as for none has been shown to me. To demon- Copernican Foscarini, opened on a conciliatory note: strate that the appearances are saved For to say that assuming the earth by assuming that the sun is at the system was moves and the sun stands still saves all center is not the same thing as to dem- the correct, the appearances better than eccentrics onstrate that in fact the sun is in the and epicycles is to speak well. … But to center and the earth in the heavens.2 physically real affirm that the sun is really fixed in the center of the heavens and that the earth Bellarmine’s letter sets the stage for a description of revolves very swiftly around the sun is challenging inquiry: What kind of evidence a dangerous thing, not only irritating convinced Galileo and Kepler that the our universe, the theologians and philosophers, but Copernican system was the correct, physi- and yet failed by injuring our holy faith and making cally real description of our universe, and yet the sacred scripture false.1 failed to convince Bellarmine? What would to convince it have taken to convince Bellarmine? For Bellarmine made very clear that he was example, most astronomy textbooks today Bellarmine? unwilling to concede the motion of the earth list the Foucault pendulum as the proof of ASA Fellow Owen Gingerich is a senior astronomer emeritus at the Smithsonian the earth’s rotation, and the annual stellar Astrophysical Observatory and Research professor of astronomy and of the parallax as the proof of the earth’s yearly history of science at Harvard University. He has served as vice president of the revolution around the sun. Would these American Philosophical Society and as chairman of the U.S. National Committee evidences have converted Bellarmine to the of the International Astronomical Union. A member of the Mennonite Congrega- Copernican doctrine, and if not (as I shall tion of Boston and an active participant in the international science-religion argue), why not? Framing the question in dialogue, he has twice given an Advent sermon at the National Cathedral in these terms will enable us to distinguish Washington. As a plenary speaker at the 2002 ASA annual meeting, he read a between proof and persuasion, and to gain chapter from his forthcoming volume, The Book Nobody Read, which has some insight into the matter of truth in an expected publication date of November 2003. Correspondence may be sent science. to him at: [email protected] 80 Perspectives on Science and Christian Faith Owen Gingerich Copernicus himself does not state directly what in itself,” but in the Copernican system, it became a induced him to work out the heliocentric arrangement, reasoned fact.4 apart from some rather vague dissatisfaction with his perceived inelegance of the traditional geocentric pattern. In the cosmological chapter 10 of Book I, Copernicus But Copernicus was nothing, if not a unifier. In the Ptole- noted that the heliocentric arrangement finally provided maic astronomy, each planet was more or less its own a natural explanation of this otherwise unexplained independent entity. True, they could be stacked one after coincidence. He mentioned as well that it explained why another, producing a system of sorts, but their motions the retrograde motion of Jupiter was smaller than that were each independent. The result, Copernicus wrote in of Mars, and why that of Saturn was still smaller. As the preface to his book, was like a monster composed of Copernicus’s only student and disciple, Georg Joachim spare parts: a head from here, the feet from there, the arms Rheticus put it: from yet another creature. Each planet had a main circle All these phenomena appear to be linked most nobly and a subsidiary circle, the so-called epicycle. Copernicus together, as by a golden chain; and each of the plan- discovered that he could eliminate one circle from each ets, by its position and order and every inequality of set by combining them all into a unified system, and its motion, bears witness that the earth moves and when he did this, something almost magical happened. that we who dwell upon the globe of the earth, Mercury, the swiftest planet, circled closer to the sun than instead of accepting its changes of position, believe any other planet. Lethargic Saturn automatically circled that the planets wander in all sorts of motions of farthest from the sun, and the other planets fell into their own.5 place in between, arranged in distance by their periods of revolution. Yet these explanations were not enough to win the day. Astronomers of the sixteenth century belonged to a long tradition that had distinguished astronomy from physics. At the universities, astronomy was taught as part of the Each planet had a main circle and a quadrivium, the four advanced topics of the seven liberal arts. The astronomer instructed his students in the celestial subsidiary circle, the so-called epicycle. circles, the geometry of planetary mechanisms, and the Copernicus discovered that he could calculation of positions required for making up horo- scopes. However, the physical nature of the heavens was eliminate one circle from each set described not in Aristotle’s De coelo, but in his Metaphysica, and that text belonged to the philosophy professor. by combining them all into a unified The distinction was clearly stated in the anonymous system, and when he did this, … [the “Introduction to the Reader,” added to De revolutionibus by the Lutheran clergyman Andreas Osiander, who had planets] arranged in distance [from the served as proofreader for the publication. He wrote (and sun] by their periods of revolution. I paraphrase): You may be worried that all of liberal arts will be thrown into confusion by the hypotheses in this book, but not to worry. It is the astronomer’s task to His monumental treatise, De revolutionibus, was pub- make careful observations, and then form hypothe- lished in the year he died, 1543. In chapter 10 of Book I, ses so that the positions of the planets can be Copernicus summed up his aesthetic vision: “In no other calculated for any time. But these hypotheses need way do we find a wonderful commensurability and a sure not be true, not even probable. A philosopher will harmonious connection between the size of the orbit and seek after truth, but an astronomer will just take the planet’s period.”3 It is the most soaring cosmological what is simplest. And neither will find truth unless passage in his entire book. The key word is commen- 6 surability, the translation of Copernicus’ symmetria (liter- it has been divinely revealed to him. ally syn = common and metria = measure). The common Osiander has been much castigated for having had the measure was the earth-sun distance, which provided the presumption to preface Copernicus’ treatise in this man- measuring rod for the entire system. ner, but he was preaching to the choir in what he added. Once this heliocentric unification was accomplished, The Protestants in Wittenberg endorsed the interpretation, the system showed other advantages. There was, e.g., the and surely would have invented it if Osiander had not curious fact that whenever Mars or Jupiter or Saturn went already clearly stated it. The Catholics likewise fell in line, into its so-called retrograde motion, the planet was always as Bellarmine’s opinion reveals. In the opening lines of his directly opposite the sun in the sky. As Gemma Frisius letter to Foscarini, he stated: “First, I say that it appears to was to describe it soon after the publication of De me that your Reverence and Signor Galilei did prudently revolutionibus, from antiquity this had been merely a “fact to content yourselves with speaking hypothetically, as I Volume 55, Number 2, June 2003 81 Plenary Presenters Truth in Science: Proof, Persuasion, and the Galileo Affair have always supposed Copernicus did.”7 knew that in the Ptolemaic system, the epicycle When Galileo was negotiating with Cosimo of Mars always lay beyond the sun, whereas de Medici for his new position in the Floren- in the Copernican arrangement, Mars at its tine court, he was comparatively indifferent closest was only half that distance away. about his salary, but he was insistent on the Because Tycho, like Copernicus and Ptolemy title: Mathematician and Philosopher to the before him, accepted an erroneously small Grand Duke.